You have a very good point here. However, this is what Jesus says in John 10:30. Jesus said "I and my Father are one". The "one" here doesn't represent "person" but "nature". Although Jesus and God the Father are 2 distinct persons, they have a common "nature". I am a Catholic so my views may represent the Catholic view. Here is what the Catholic Church says about the 3 distinct persons with a common nature:

John 1:1 "In the beginning was the Word. the Word was with God and the Word was God" If Jesus is the Word of God then this verse shows that Jesus was there in the beginning with God the Father which makes Jesus divine and eternal just like God the Father.
From Mic. 5:2 "the Messiah(Jesus) had always been in existence". Also, take a look at John 5:18 "The Jews wanted to kill Jesus because He was making himself equal with God"
Acts 5:3,4 ""… you have lied to the Holy Spirit … you have not lied to men, but to God." also supports the idea that the Holy Spirit is God.
Also read 1Cor 12:4-31
"There are different kinds of gifts, but the same Spirit. There are different kinds of service, but the same Lord. There are different kinds of working, but the same God works all of them in all men"...
There are many more verses in the Bible supporting the idea that Jesus is One with God and also the idea that the Holy Spirit is God. There are 3 distinct persons with a common nature and that nature is what we call God. In other words, there is just
ONE God but there are
3 distinct persons in this ONE God